Which makes sense because there's nothing casual about the experience of being tickled, whether it turns you on or not -- it's an all-encompassing, all-consuming sensation; being ticklish means other people have the power to make you drop whatever you're doing, forget whatever you're thinking, instantaneously, and swallow your consciousness up in this completely incapacitating sensation, one that manifests in the exertion of full-throated laughter. It's counterintuitive and a little bizarre -- it's as though other people had the power to, with the motion of a single finger, force you to start hiccuping, or start exercising -- except that the spectacle is more entertaining than that, which makes other people more likely to do it, and to keep doing it, because who doesn't enjoy other people's laughter?
It's like having a body covered all over with defcon-1 red-alert buttons so that other people can, with very little effort, throw you into fits. It's no wonder that people who enjoy it really really enjoy it -- and, conversely, that people who don't like it really really don't like it.
